A primary concern among users of modern electronic devices is power consumption. For example, the length of time that a user may operate a particular portable electronic product before recharging the associated battery is generally a major purchase consideration for most consumers. Consequently, a major goal of most electronics designers is to reduce the power consumption of their product designs.
Toward this end, many electronic component manufacturers provide integrated circuits (ICs) with a "low-power" mode in addition to their normal "high-power" state. (Other designations for lower power states exist, such as "standby" or "sleep" mode. Additionally, many ICs implement several different low-power levels, each with its own level of functionality and power consumption.) The electronics designers incorporate these ICs into their product designs, utilizing the low-power mode of the ICs prudently so that the products consume the least amount of power possible while still providing the functionality and performance that the user expects.
In general, placing an IC into a low-power state results in the primary functionality of the IC being disabled. During low-power mode, many circuits internal to the IC, including high-frequency clock signals and input/output (I/O) drivers, are essentially turned off, thereby dissipating very little electrical power. Once it is determined that the IC needs to perform a function that requires low-power mode to be terminated, a "wake up" signal is used to end low-power mode, thus bringing the circuit to a fully functional, high-power state.
One common example of a wake-up signal is the output of a time delay circuit that is triggered at the time a portion of the IC is placed in low-power mode. After a desired length of time that a portion of the IC has been in low-power mode, the time delay circuit generates a wake-up signal that returns the low-power portion of the IC to its normal high-power state. A time delay circuit is typically used in this role when a portion of IC circuitry in low-power state must be awakened periodically to perform a specific function before returning to low-power mode. Often, such a time delay circuit is implemented, as shown in FIG. 1, by a precision resistor R.sub.1 in series with a precision capacitor C.sub.1, configured as a low-pass filter. External precision components are normally used since the accuracy of the time delay circuit is somewhat important for many electronics applications. The time constant of this circuit, along with the threshold voltage of the IC input acting as the wake-up signal input, determines the time delay (the delay between voltage V.sub.start going high and voltage V.sub.delay going high) until the low-power portion of the IC is awakened. Unfortunately, such a solution requires relatively expensive external components, valuable space on the associated circuit board to house the components, and an IC package pin dedicated to the wake-up signal.
To eliminate the need for an extra IC package pin, expensive external components, and additional circuit board space for the wake-up function, the circuit that generates the wake-up signal could be implemented on the low-power-capable IC. For example, an internal timer circuit driven by a highly accurate onboard crystal or resonator oscillator that generates a stable accurate clock signal may be used for such a purpose. However, such an accurate oscillator generally consumes a few tens to several hundreds of milliamps of current, thus making the accurate oscillator a high-power circuit that should be turned off in low-power mode. Additionally, the superb accuracy of such oscillators is simply not necessary when applied to a wake-up time delay circuit. Other types of oscillator circuits, such as ring oscillators or relaxation oscillators, may be utilized to drive the time delay circuit. Unfortunately, such low-power oscillators are rather unstable and inaccurate, their operating frequency varying by as much as a factor of two- or three-to-one over changes in IC process, supply voltage, and operating temperature. For the majority of IC applications, such frequency variations of inaccurate oscillators are not acceptable for a wake-up time delay circuit.
Therefore, it would be advantageous to implement a time delay circuit utilizing an inaccurate oscillator that generates an accurate time delay signal.